This invention relates to a power-on reset circuit and more particularly to the structure of such a circuit. This invention additionally relates to a one-chip microcomputer using such a power-on reset circuit.
With a control unit comprising a prior art one-chip microcomputer used, for example, in acoustical or image-forming apparatus, the product as a whole frequently functions incorrectly or there is a runaway situation when the source voltage drops momentarily during the running of a program and returns thereafter to its normal level due, for example, to a power failure or a noise, as will be discussed below, because the internal condition of the control device after such an event is usually uncertain. In view of occurrences of this type, it has been known to use a power-on reset circuit, either incorporated internally or attached as an external device, to a semiconductor device such that the internal condition of the control device will be reset to a specified initial condition when the source is restored.
FIG. 5 shows an example of prior art power-on reset circuit 12 incorporated in a semiconductor device, and FIG. 6 is its waveform diagram. In what follows, the source voltage VDD will be sometimes referred to as "H", or the high level and the reference voltage as "L" or the low level.
With reference simultaneously to both FIGS. 5 and 6, capacitors C1 and C2, each having one of its electrodes maintained at the reference voltage (GND), are in a discharged condition before the source voltage (VDD) is applied to them at time t1, and their voltage is generally at the reference voltage "L". After the source voltage is applied at time t1, the voltage of capacitor C2 remains at the reference voltage "L" because of the principle of charge conservation, but the output signal RST of the voltage detecting circuit 1 becomes "H", thereby resetting the inner circuits of the semiconductor device (not shown). Thereafter, control signals CK1 and CK2 and their inversion signals CK1 and CK2 are alternately inputted as shown in FIG. 6 to analog switch circuits S1 and S2, respectively, from a timing circuit (not shown). Whenever either of the signals CK1 and CK2 is "H", the corresponding switch circuit S1 or S2 becomes conductive. When signal CK1 is "H" and signal CK2 is "L", for example, switch circuit S1 alone becomes conductive, causing capacitor C1 to be charged by the source voltage. When signal CK1 is "L" and signal CK2 is "H", on the other hand, switch circuit S2 alone becomes conductive, causing the charge on capacitor C1 to become distributed between the two capacitors C1 and C2.
As this process is repeated, the voltage VC2 of capacitor C2 gradually increases, approaching the source voltage. When, at time t2, the voltage VC2 of capacitor C2 finally exceeds the input (logic) threshold voltage VT of the voltage detecting circuit 1, the output voltage of the voltage detecting circuit 1 is inverted, becoming "L". The reset condition is thereby released, and the circuit is now ready for a normal operation. Thus, the power-on reset circuit 12 of FIG. 5, although compactly structured so as to be incorporated in a semiconductor circuit, is capable of providing a fairly long reset time without using any capacitor with a large capacitance.
A momentary drop in the source voltage, lasting several tens of milliseconds or less, sometimes takes place, as shown as happening at time t3 in FIG. 6, due, for example, to an external noise by a surge voltage applied externally of the semiconductor device or an internal noise generated within the semiconductor device such as when the internal circuits are activated together or when an output circuit drawing a large current was activated. When such a momentary drop takes place, the power-on reset circuit 12 instantaneously discharges the voltage charged to capacitor C2 through the diode D1 until the forward voltage VF of the diode D1 is reached, discharging gradually thereafter through leak currents or the like until the reference voltage is reached. If the voltage VC2 of capacitor C2 drops below the input threshold voltage VT even momentarily in such a situation, the RST output becomes "H" and the internal circuit is reset.
For this reason, even if the source voltage remains high enough to be able to keep the data in data-storing means for the semiconductor device, they are reset to their initial conditions, and the program operation cannot be resumed from the condition immediately prior to the momentary voltage drop.